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Full text of "The Cambridge Natural History"

§4                            TKETH OF CAFE AtfTEATER                         CHAP.
same phenomenon; for Sir W. Flower showed, and Mr. Thomas
confirmed his discovery, that only one tooth, according to Mr.
Thomas the fourth premolar, is replaced in that group. But
even the purely monophyodont dentition of the Toothed Whales
is a more apparent than real contrast to the diphyodout dentition
elsewhere prevalent. An investigation of the embryos of various
'Toothed Whales by Dr. Kiikenthal and by Dr. Leche has brought
to light the highly important fact that two dentitions are present,
but that one only comes to maturity; from this fact obviously
follows the interesting question :—To which of the two dentitions
of more normal Mammalia does the monophyodont dentition of the
Whales and Marsupials belong ? To this question a clear answer
is fortunately possible. As has been pointed out in the fore-
going sketch of tooth development, and has been illustrated in
the figures, the milk teeth develop as lateral outgrowths of the
common enamel germ, while the permanent teeth arise from the
end of the same band of tissue. This fact enables it to be
stated apparently beyond a doubt that in the Whales and in the
Marsupials it is the milk dentition which is the only one to
arrive at maturity. Thus the earlier theoretical conclusion that
the Marsupial dentition " is a secondary dentition with only one
tooth of the primary set left," is proved on embryological grounds
to be untrue. But there are other inonophyodont animals than
those already mentioned.1 Orycteropus, the Cape Anteater, is an
example. Mr. Thomas has lately discovered that in this Eden-
tate there is a set of minute though calcified milk teeth which
probably never cut the gum; here we have a different sort of
monophyodontism, in which the teeth belong to the second and
not to the first set. Between the latter condition and the
diphyodont state are intermediate stages. Thus in the Sea Lions
the milk teeth are developed but disappear early, probably before
the animal is born.
In the typical diphyodont" dentition, such as is exhibited for
example in Man and the vast majority of mammals, the milk teeth
eventually completely disappear and are entirely replaced by the
permanent set of teeth, with the exception, of course, of the molars,
which though they are developed late belong to the milk series.
1 It would T>e of the greatest interest in relation to this and many oth«r
problems to ascertain the precise meaning of tlie monopltyodont dentition of
Ornithorhyiicfcua.